1. Field of the Invention
The present invention relates to a thermoplastic blend and to a compressible package of the tube type, obtained using this blend. The invention relates more particularly to the production, by injection moulding, of plastic tubes having an improved stress-cracking resistance. The package of the invention is intended, in particular, to contain a cosmetic, pharmaceutical, dermo-pharmaceutical product or certain food products, and is particularly suitable for containing a product which contains one or more surfactants.
2. Discussion of the Background
Stress cracking is a term used to describe a phenomenon of chemical attack of a product acting on a polymeric material. The phenomenon is almost imperceptible when this material is not subjected to a mechanical stress. However, contact of the material with the product causes microcracks to form. When the material is then put under stress--for example, in the case of a plastic bottle containing a shampoo, by manually compressing the closed bottle--the microcracks propagate, leading to embrittlement of the material and to fracture of the polymeric wall. In the example in question, the bottle will therefore inevitably leak. The stress-cracking phenomenon occurs in particular in strongly folded regions or in heat-sealing regions, for example at the bottom of a tube which has been obtained by pinching the free end of the compressible wall of the tube, followed by heat sealing.
Currently, most of the packages used are made of plastic, particularly in the cosmetic field, such as tubes or bottles intended to contain a cream or a shampoo. In order to manufacture them, thermoplastic polymers, and more particularly suitable polyolefins, are generally used, prepared by an extrusion or extrusion blow-moulding process with the aid of conventional industrial equipment.
Among the known thermoplastic polyolefins used are low-density polyethylenes, intended in particular for the manufacture of tubes. Low-density polyethylene tubes are flexible, thereby allowing easy expulsion of the product contained in the tube by simply compressing it. However, this type of polyethylene is incompatible with many chemicals and the phenomenon of stress cracking is regularly observed in the case of low-density polyethylene tubes, in particular when they contain a product comprising one or more surfactants. As a consequence, it is not conceivable to use such a tube for packaging a shampoo or a body-care emulsion, which generally contain at least one surfactant. This stress-cracking phenomenon is even more pronounced when the tubes are produced by injection moulding.
A thermoplastic blend that can be used for producing articles resistant to stress cracking is known, for example, from document EP-A-0,530,782, this blend being composed of:
a) a linear low-density ethylene copolymer containing a C.sub.4 to C.sub.8 olefin having approximately 10 to 15 long-chain branches per 1000 carbon atoms and PA1 b) a low-density polyethylene, PA1 (a) a first ethylene/C.sub.4 to C.sub.5 olefin copolymer and PA1 (b) a second ethylene/C.sub.6 to C.sub.10 olefin copolymer,
with the copolymer being present in the blend in a proportion of between 25% and 35%. According to the illustrative embodiments, this blend is used for the manufacture of articles moulded by the techniques of extrusion or of blow moulding. The melt flow indices of the materials mentioned in this reference do not allow these materials to be used for producing tubes using the technique of injection moulding.
Furthermore, EP-A-0,103,942 describes bags manufactured from a plastic film, the film being made of a blend comprising at least one ethylene/C.sub.6 to C.sub.10 olefin copolymer having a density of between 0.916 and 0.930 g/cm.sup.3 and an ethylene/C.sub.4 to C.sub.10 olefin copolymer having a density of between 0.916 and 0.924 g/cm.sup.3. This blend cannot be used for the injection moulding of thin-walled articles since, at the temperatures of injection moulding, it has too low a flow index.
Likewise, blends of ethylene/octene copolymers and ethylene/butene copolymers described in document EP-A-0,315,028 cannot be used since they have too low a flow index at the injection-moulding temperatures.